Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC. LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristics of a polishing pad, it is requested that a polished object is excellent in planarity and within wafer non-uniformity and a polishing rate is large. A planarity and within wafer non-uniformity of a polished object can be improved to some extent with a polishing layer higher in elastic modulus. A polishing rate can be bettered by increasing a holding quantity of a slurry on a foam with cells therein.
Polishing pads including a polyurethane foam are proposed as polishing pads that meet the above properties (see Patent Documents 1 and 2). Such a polyurethane foam is produced by a reaction of an isocyanate-terminated prepolymer with a chain extender (curing agent), in which in view of hydrolysis resistance, elastic properties, wear resistance, or the like, a polyether (a polytetramethylene glycol with a number average molecular weight of 500 to 1,600) or a polycarbonate is preferably used as a high molecular polyol component for the isocyanate prepolymer.
Generally, when planarization of a large number of semiconductor wafers is performed using a polishing pad, a fine uneven portion of the surface of the polishing pad is worn to deteriorate the performance of supplying the polishing agent (slurry) to the surface to be processed of the semiconductor wafer, decrease the speed of planarizing the surface to be processed of the wafer, or deteriorate the planarization characteristics. Therefore, after having performed the planarization of a predetermined number of semiconductor wafers, it is necessary to renew/roughen (dress) the surface of the polishing pad by using a dresser. When dressing is carried out for a predetermined period of time, uncountable fine uneven portions are produced on the surface of the polishing pad, so that the surface of the polishing pad becomes fluffy.
However, the conventional polishing pad has a problem that the dressing speed during dressing is low and such dressing takes too much time.
In order to solve the above problem, Patent Document 3 proposes a technique of using a polymerized diisocyanate and an aromatic diisocyanate as an isocyanate component that is a starting material for polyurethane resin foams.
However, there is a tendency such that when a polymerized diisocyanate is used, the hardness of the polyurethane resin foam increases and when a polishing pad formed of the polyurethane resin foam is used, scratches are likely to occur on the surface of an object to be polished.
Further, as a polishing pad satisfying the above characteristics, Patent Document 4 proposes a polishing pad that is formed of an unfoamed synthetic resin.
The polishing pad that is formed of an unfoamed body has a problem that the cut rate is low at the time of dressing and that the dressing takes too much time.
In order to solve this problem, Patent Document 5 proposes a polishing pad having a polishing layer that is formed of an unfoamed polyurethane, wherein the unfoamed polyurethane is a reaction hardened body of a polyurethane starting material composition which contains an isocyanate-terminated prepolymer obtained by the reaction of a prepolymer starting material composition containing a diisocyanate, a high molecular weight polyol, and a low molecular weight polyol; a modified isocyanate which is polymerized by the addition of three or more diisocyanates; and a chain extender, and the amount added of the modified isocyanate being 5 to 30 parts by weight per 100 parts by weight of the isocyanate-terminated prepolymer.
The polishing pad is one which hardly causes scratches on the surface of an object to be polished while having improved dressing properties, but further improvement thereof will be required in the future.